1. Field of the Invention
Embodiments of the invention relate generally to a method of forming a protection layer on a TEM specimen and a related method of preparing a specimen for transmission electron microscope (TEM) inspection.
This application claims priority to Korean Patent Application No. 2004-72456 filed on Sep. 10, 2004, the subject matter of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
In general, a semiconductor device is formed by a complex sequence of processes, including one or more patterning process(es). Conventional patterning processes are adapted to form, for example, a circuit pattern on a semiconductor substrate. The patterning process may include an etching process, a diffusion process, and a metallization process. Recent trends in the use of semiconductor devices are characterized by demands for multi-functionality and high speed performance. In order to meet these demands, semiconductor devices are ever more densely integrated. Increasing integration density demands tighter tolerances on circuit patterns. In order to meet these tighter design tolerances, a greater emphasis has recently been placed on inspection and analysis (e.g., structural and/or chemical) processes and equipment implementing these processes. The transmission electron microscope (TEM) is one such piece of equipment increasingly adapted to the inspection and analysis of circuit patterns formed on semiconductor substrates. Indeed, use of the TEM is wide spread due to its high resolution and general applicability to inspection and analysis processes.
However, while the TEM provides very accurate information about an object under inspection, the inspection and analysis results provided by the TEM are heavily dependent on the quality of the exemplary specimen associated with the object. That is, TEM inspection of an entire object or structure is rarely possible under commercial fabrication circumstances, so an exemplary specimen must be prepared. Accordingly, special emphasis is laid upon a related method of preparing the specimen for TEM inspection.
Conventional TEM specimens formed to inspect a predetermined layer on a semiconductor substrate have usually been prepared using an ion milling process or a focusing ion beam (FIB) process.
In the conventional FIB process, an etching process is initiated from a neighboring point proximate the inspection point on the specimen and then moved towards the inspection point during a period in which the inspection point is being viewed as an electronic image. Using this technique, the thickness of the specimen may be easily controlled.
Examples of conventional methods used to form TEM specimens include those disclosed in U.S. Pat. No. 6,194,720 to Li et al., and U.S. Pat. No. 6,080,991 to Tsai. Li et al. disclose a method of forming a TEM specimen including first and second electron transparent segments. The first electron transparent segment is formed using a FIB technique while the second electron transparent segment is formed using a wedge forming technique. Tsai discloses a method of forming the TEM specimen comprising a thin layer taken from a portion of the object or structure being inspected.
According to yet another conventional method of preparing a TEM specimen; an inspection point—which contains a defect—is first identified (or selected) using an electronic image. Then, the semiconductor wafer is cut into a wafer slice of about 2 mm×3 mm using an ultrasonic wave cutter such that the inspection point is positioned in a middle of the specimen, thereby forming a first specimen.
The first specimen is then ground to a thickness of about 40 μm, thereby forming a second specimen. The second specimen is adhered to a nickel grid, and a protection layer is coated over the inspection point using a deposition process, thereby forming a third specimen. Both side surfaces of the third specimen are then milled by sequentially varying the current applied to a FIB process, thereby forming the TEM specimen having a vertical side surface suitable for inspection.
FIG. 1 is a cross sectional view illustrating a conventional method of forming a protection layer on a TEM specimen. More particularly, FIG. 1 illustrates one step in the foregoing conventional method adapted to the preparation of the first specimen.
Referring to FIG. 1, a first layer of protection material is deposited on a stage 12 and a wafer 10 (e.g., a preliminary specimen cut from a semiconductor substrate) is placed on the first layer of the protection material. Then, a second layer of the protection material is deposited on wafer 10. Thus, the first and second applications of the protection material form a protection layer 14 on wafer 10. A covering member 16 is then positioned on protection layer 14. Thereafter, wafer 10 is cut into a wafer slice using an ultrasonic wave cutter.
As shown in FIG. 1, although top and bottom surfaces of wafer 10 are covered by protection layer 14, the side surface(s) of wafer 10, including a beveled side surface, are barely, if at all, covered by protection layer 14. As a result, when wafer 10 is cut into a wafer slice by the ultrasonic wave cutter, the beveled side surface of wafer 10 may be broken off or otherwise removed. Accordingly, where an inspection point P is located on a beveled side surface of wafer 10, the ultrasonic wave may cause damage to inspection point P (i.e., the point of interest). When inspection point P is damaged by the ultrasonic wave, the specimen is no longer useful and a new specimen must be prepared, since TEM inspection of a broken or damaged specimen if of little value. This outcome is particularly harmful where the inspection point P is associated with a unique defect or point of interest.
FIG. 2 is a cross sectional view illustrating one conventional method of grinding a specimen being prepared for TEM inspection. More particularly, FIG. 2 illustrates one step in the foregoing conventional method adapted to the preparation of the second specimen.
Referring to FIG. 2, an inspection point P is again located on a beveled side surface of a first specimen 20. The beveled side surface is secured to a stage 22 using an adhesive and a covering member 26 is fixed to the vertical lateral side of the secured first specimen 20. During the grinding process, the specimen may be laterally visualized through covering member 26. Unfortunately, inspection point P cannot be viewed through covering member 26 during the grinding process, and the grinding process must be guided solely by an operator's intuition. Accordingly, the personal skill of an operator has a large effect on the grinding quality, and thus the reliability of the grinding process may be relatively low.